TM 5-809-1/AFM 88-3, Chap. 15
CHAPTER 2
BASIS OF FLOOR SLAB ON GRADE DESIGN
2-1. Stresses.
thermal expansion and contraction of the concrete
slab and warping stresses resulting from moisture
The structural design of a concrete floor slab on
and temperature gradients within the slab, due to
grade is primarily controlled by the stresses caused
their cyclic nature, will at times be added to the
by moving live loads and in some cases the
moving live load stresses. Provision for these
stationary loads. Stresses in floor slabs on grade
stresses that are not induced by wheel loads is made
resulting from vehicular loads are a function of floor
by safety factors developed empirically from full-
slab thickness, vehicle weight and weight
scale accelerated traffic tests and from the observed
distribution, vehicle wheel or track configuration,
performance of pavements under actual service
modulus of elasticity and Poisson*s ratio of
conditions.
concrete, and modulus of subgrade reaction of
supporting material. The volume of traffic during
2-3. Stationary live loads.
the design life is important for fatigue consid-
erations. The floor slab design procedure presented
The maximum allowable stationary live load is lim-
herein is based on limiting the critical tensile stresses
ited by both the positive bending moment stress
produced within the slab by the vehicle loading, as
under the load and the negative bending moment
in TM 5-822-6/AFM 88-7, Chap. 1. Correlation
stresses occurring at some distance from the load.
a. Positive bending moments. Stresses due to
studies between theory, small-scale model studies,
and full-scale accelerated traffic tests have shown
positive bending moment are relatively simple to
compute by using Westergaard*s analysis* of elasti-
that maximum tensile stresses in floor slabs will
occur when vehicle wheels are tangent to a free
cally supported plates. An appropriate safety factor
edge. Stresses for the condition of the vehilcle
is applied to determine allowable stresses due to
wheels tangent to an interiorjoint where the two
these loads because environmentally imposed
slabs are tied together are less severe than a free
stresses must also be accounted for when
edge because of the load transfer across the two
considering stationary loads.
b. Negative bending moments. The effect of
adjacent slabs. In the case of floor slabs, the design
can be based on the control of stress at interior
negative bending stress is somewhat more difficult
joints. Exceptions to this assumption for interior
to determine. A slab on an elastic subgrade will
joint loading occur when a wheel is placed at the
deform under loading somewhat like a damped sine
edge at doorways or near a free edge at a wall.
curve in which the amplitude or deformation of
successive cycles at a distance from the loading
2.2 Vehicle-imposed loads.
position decreases asymptotically to zero. Thus,
there exists some critical aisle width where the
For determining floor slab design requirements, mili-
damped sine curves from parallel loading areas are
tary vehicles have been divided into three general
in phase and additive. In this situation, the negative
classifications: forklift trucks, other pneumatic and
bending moment stresses wil become significant and
solid tired vehicles, and tracked vehicles. The
must be considered. Therefore, allowable stationary
relative severity of any given load within any of the
live loads were established to include the effects of
three classifications is determined by establishing a
negative moment bending stresses. These
relationship between the load in question and a
calculations are reflected in the tabulated values of
standard loading. Floor slab design requirements are
allowable stationary live loads.
then established in terms of the standard load. Other
stresses such as restraint stresses resulting from
* Westergaards analysis Is actually for plates on a liquid foundation, sometimes called a Winkler foundation. There Is a distinct
difference between the structural behavoir of plates on a liquid and on an elastic foundation. In many textbooks, the term "beam
on elastic foundation" Is actually "beam on liquid foundation."
2-1
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